We have reconstructed the sequence of a human gene (referred to as an L1 element) that has features which resemble the transposable elements of yeast and Drosophila. This gene has given rise to a very large class of copies that constitute as much as one percent of the human genome. The sequence contains two open reading frames, one with homology to possible DNA binding proteins and the other to the reverse transcriptases of various retroviruses and transposable elements. Because of its extraordinary ability to retrotranspose (and possibly transpose) this sequence may make an excellent vector for the integration of specific genes into the genome. On the basis of our consensus sequence we propose to construct a functional human L1 element by "correcting" deleterious substitutions that have occurred in a processed pseudogene element that we have sequenced. Restriction sites will also be modified in the sequence and a polylinker cloning region will be added to permit the insertion of various target sequences. Functional testing of the vector will be carried out by cloning a selectable marker (HPRT) and transfecting cultured mouse and human cells.